Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit often includes a focal plane array of pixels as an image sensor, with each pixel including a photosensor which may be a photogate, a photoconductor, a photodiode, or other device having a doped semiconductive region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region formed on or in the substrate which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device, such as a transistor, for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level. Further and regardless, in CMOS and other imaging devices, some components of a pixel might be shared with other pixels.
In a CMOS imaging device, the active elements of a pixel perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imaging devices of the type discussed above are generally known and discussed, for example, in U.S. Pat. Nos. 6,140,630; 6,376,868; 6,310,366; 6,326,652; 6,204,524; and 6,333,205.
In order to capture images with greater resolution while also maintaining a small image sensor, it is desirable to design image sensors with a large number of relatively small pixels. As pixels become smaller, however, many of the transistors responsible for reading out the pixel signal cannot practically be made smaller, and begin to take up most of the space allocated to a single pixel. Consequently, the photosensor of the pixel becomes smaller while more of the pixel area is used by the pixel transistors such that the pixel fill factor, which is the percentage of a pixel that is photosensitive, is reduced. As photosensor size and pixel fill factor shrink, the amount of light that is converted to a signal within each pixel decreases as well.
Further, each pixel encompasses multiple metal routing layers typically formed above the photosensor and transistor gates, and which are used to convey signals, e.g., control signals, for the readout circuits between the various transistors of the pixel. As pixels are made smaller, these metal routing layers become more obstructive to light that would otherwise reach the pixel photosensor. When connecting, for example, a Metal 2 level line to a pixel component received below the Metal 1 level routing layer, an electrically isolated Metal 1 island is used as a conductive interconnect between a conductive via from Metal 2 to the island and a conductive contact from the Metal 1 island to the pixel component therebelow. The conductive Metal 1 island can require spacing of Metal 1 lines within the pixel further apart, thereby reducing the metal opening size over the photodiode of an individual pixel.